1. Field of the Invention
The present invention relates to tank tightness testers for measuring the tightness of tanks, including, but not limited to, underground fuel storage tanks, and the method of performing the test, and more particularly relates to a microprocessor controlled leak tester for such tanks.
2. Background Information
Applicant's assignee is the successor in interest to Kent-Moore Corporation of Warren, Michigan, the assignee of U.S. Pat. No. 3,580,055 entitled "TANK SYSTEM TIGHTNESS TESTER". This patent is the closest prior art of which we are aware. The purpose of the present invention is to automate tank testing, and at the same time to do so according to a method which makes such tank testing even more accurate than previously possible by taking into account factors that were not fully realized in the development of previous tank tightness testers.
While said earlier U.S. Pat. No. 3,580,055 appreciated the need to stabilize temperature within the tank being tested, the inventor therein did not fully realize the effect which deflection of the tank ends has on tank volume. The pressure exerted by the fluid in a tank deflects mainly the ends of the tank, there being little deflection, if any, on the sides of the tank. But the pressure exerted on those ends is considerable. For example, gasoline in a tank having a 10 foot diameter will exert 3.16 tons of pressure on the ends of the tank, and gasoline in a 15 foot diameter tank will exert 4.74 tons on the end of the tank. The pressure will of course vary with the specific gravity of the fluid contained.
If the ends of the tank do not abut up against a solid surface, such as a concrete container for example, but instead are open to the air, or in the example being used for illustration in the present application, are buried underground, the tank ends will deflect under said aforementioned pressure until a stabilized condition is reached because the only resistance to said deflection is the ground water pressure.
Such stabilized condition takes some time to occur, and in the time available for testing can only be closely approximated. Thus, it is desirable to perform the tank tightness test as quickly as possible. In the prior art there are known various methods for attaching a standtube to the inlet of an underground tank, and filling such standtube, for example, to 40 inches above the ground level to purposely create a head greater than one wants during the actual test, and to create a large rate of deflection of the ends of the tank, then the level is lowered in the standtube and kept at this level to have the tank at a nearly zero rate of deflection.
Failure to take into account the expansion of the tank will give a false indication of a leak. For example, a tank having a diameter of 96 inches will initially have a deflection of 0.125 inches, which results in an apparent loss of 3.92 gallons of fuel. It is, therefor, absolutely necessary to compensate for the deflection of the walls of a tank when calculating the change in the volume in a tank during a test procedure, or considerable time will be required before equilibrium is reached.
However, many of the prior art methods, including the aforementioned U.S. Pat. No. 3,580,055, use a method which determines the change in the height of a fluid column within a standtube connected to the tank at predetermined time intervals, and the temperature is simultaneously measured within the tank at the same time intervals, and a temperature correction factor alone is used to determine if the tank is tight. This method completely ignores the influence which the continued expansion of the tank ends, albeit at a small rate, has on the tank volume. The present invention not only continuously circulates the fluid in a tank and continuously compesates for any change in temperature therein, but also continuously monitors and keeps the fluid in a standtube at a preset level, measuring any fluid added to, or taken from the standtube, to keep the tank whose tightness is being measured in as close to an equilibrium condition as is possible, so as to absolutely minimize the effects of tank end deflection on a tank volume measurement.